The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device which is capable of performing both of a transmissive display and a reflective display using a transmissive-type liquid crystal display panel.
For example, with respect to a liquid crystal display device which is served for a mobile telephone or the like, there has been known a liquid crystal display device which can perform both of a transmissive display and a reflective display such that the transmissive display is performed using light from a backlight under a dark environment and the reflective display is performed using an external light under a bright atmosphere.
The liquid crystal display device which is capable of performing both of the transmissive display and the reflective display is roughly classified into two types of systems.
The first type of system is a system which combines a liquid crystal display panel of a semi-transmissive type (also referred to as a partial transmissive type) which incorporates a semi-transparent reflective film such as a half mirror or a reflection film having an opening for allowing light from a backlight to pass therethrough in the inside of the liquid crystal display panel and the backlight.
The second type of system is a system in which a liquid crystal display panel per se uses a transmissive type liquid crystal display panel and enables a transmissive display using light from a light source such as a backlight and a reflective display which uses an external light incident from a front face side of the liquid crystal display panel in such a manner that the external light is reflected on a reflector of the backlight (see following patent literatures 1, 2 and 3).
FIG. 9 is a cross-sectional view for explaining an example of a conventional second-type liquid crystal display device. This drawing depicts a content described in the patent literature 1 in a simplified form.
As shown in FIG. 9, a backlight is arranged at a back face of a transmissive type liquid crystal display panel (liquid crystal display element).
The liquid crystal display panel is constituted such that a liquid crystal layer 3 is sandwiched between a pair of substrates (first substrate 1 and second substrate 2) which are laminated to each other using a sealing material 4, a polarizer 5 is laminated to a back face of the first substrate 1 which is positioned at a back face side, and a polarizer 6 is laminated to a front face of the second substrate 2 which is positioned at a front face side. Here, electrodes, orientation films, color filters and the like are omitted from the drawing.
The backlight is constituted of a light source 9, a light guide body 7 on which light from the light source 9 is incident and a reflector 8 which is arranged on the back face of the light guide body 7. Here, although the light guide body 7 includes a prism portion, the prism portion is omitted from the drawing. Further, although the backlight may be provided with a prism sheet, the prism sheet is omitted from the drawing.
Here, the liquid crystal display panel is of a transmissive type liquid crystal display panel and performs a transmissive display using light 20 from the light source 9. Further, the liquid crystal display panel is capable of performing a reflective display using a reflection light 22 obtained by reflecting an external light 21 from the front face side of the liquid crystal display panel on the reflector 8 of the backlight.
Further, in this liquid crystal display device, for the purpose of obtaining a display with no luminance irregularities, a light diffusion plate 10 is arranged between the liquid crystal display panel and the backlight whereby light 20 from the light source 9, the external light 21 and the reflection light 22 are diffused.
Further, in the patent literature 2, there is disclosed a liquid crystal display panel in which in place of the light diffusion plate 10, a resin film in which scattering fillers are dispersed is laminated to a lower face of the polarizer 5 at the back face side. With respect to the position of the resin film, there is a description that the resin film is arranged at an arbitrary position so long as the resin film is disposed between the reflector 8 of the backlight and the front-face-side polarizer 6. Further, there is also a description that using a diffusion layer formed of a tacky adhesive material in which scattering fillers are dispersed, members which are arranged in front of and behind the diffusion layer (for example, a reflection polarizer and a λ/4 phase plate or a twisted phase plate) may be laminated to each other. This diffusion layer is also provided for obtaining a display having no luminance irregularities.
Further, in the patent literature 3, there is disclosed a liquid crystal display panel in which, in place of the light diffusion plate 10 of the patent literature 1, a filler mixed type diffusion plate or a diffusion plate formed of a micro lens sheet is arranged between the front-face-side second substrate 2 and the polarizer 6. Further, by imparting the backscattering property to the diffusion plate, a portion of the external light 21 incident from the front face side is made to directly return to the front face side due to reflection without making the portion to pass through the liquid crystal layer 3 whereby the luminance of a screen is enhanced.
Here, the patent literature 3 discloses, as a modification of the invention described therein, an example in which a diffusion plate is arranged between the back-face-side first substrate 1 and the polarizer 5 or an example in which diffusion plates are arranged between the front-face-side second substrate 2 and the polarizer 6 as well as between the back-face-side first substrate 1 and the polarizer 5. However, in the patent literature 3, there is a description that when the diffusion plate is arranged at the back face side, light for enhancing the luminance of screen passes through the liquid crystal layer 3 and hence, a quantity of light which is irradiated to the front face side is changed in response to an image to be displayed whereby it is preferable to arrange the diffusion plate at the front face side.
Patent literature 1: Japanese Unexamined Patent Publication 2002-98960 (paragraphs 0033 to 0043, FIG. 1 to FIG. 5)
Patent literature 2: Japanese Unexamined Patent Publication 2002-98963 (paragraphs 0044, 0060, 0130 to 0132, FIG. 1, FIG. 3 and FIG. 5)
Patent literature 3: Japanese Unexamined Patent Publication 2001-91943 (paragraphs 0037, 0073 to 0076, 0087 to 0089, FIG. 1 to FIG. 5)